As a background technology for the present technical field, there is JP 2011-034775 A (Patent Literature 1). In this publication, it is described that “a separator includes an upper surface holding portion holding an upper surface, a pair of side surface holding portions holding side surfaces, and a lower surface holding portion holding a lower surface of square batteries, and an insulating portion extending between the square batteries. The lower surface holding portion, which is positioned on the cooling surface side, is formed with a cutout portion exposing surfaces of the square batteries on the cooling surface side except for a pair of corner portions in the surfaces on the cooling surface side of the square batteries. A heat-conducting sheet is abutted on a cooling surface of the assembled battery via the cutout portion, with a cooling plate abutted on the heat-conducting sheet (see the Abstract).